A. Field of the Invention
The present invention relates to treated mixed matrix polymeric membranes in which metal-organic frameworks (MOFs) such as zeolitic imidazolate frameworks (ZIFs) are attached to the polymers (e.g., by covalent bonding) such that voids between the interface of the MOFs and polymers are reduced in number or size or both. Further, the membranes can be treated with plasma, electromagnetic radiation, or thermal energy, or any combination thereof. This combination of the attachment of the MOFs to the polymers of the membrane along with the surface treatment of the membranes results in polymeric membranes that have surprisingly improved selectivity parameters for gas separation applications.
B. Description of Related Art
A membrane is a structure that has the ability to separate one or more materials from a liquid, vapor or gas. The membrane acts like a selective barrier by allowing some material to pass through (i.e., the permeate or permeate stream) while preventing others from passing through (i.e., the retentate or retentate stream). This separation property has wide applicability in both the laboratory and industrial settings in instances where it is desirable to separate materials from one another (e.g., removal of nitrogen or oxygen from air, separation of hydrogen from gases like nitrogen and methane, recovery of hydrogen from product streams of ammonia plants, recovery of hydrogen in oil refinery processes, separation of methane from the other components of biogas, enrichment of air by oxygen for medical or metallurgical purposes, enrichment of ullage or headspace by nitrogen in inerting systems designed to prevent fuel tank explosions, removal of water vapor from natural gas and other gases, removal of carbon dioxide from natural gas, removal of H2S from natural gas, removal of volatile organic liquids (VOL) from air of exhaust streams, desiccation or dehumidification of air, etc.).
Examples of membranes include polymeric membranes such as those made from polymers, liquid membranes (e.g., emulsion liquid membranes, immobilized (supported) liquid membranes, molten salts, etc.), and ceramic membranes made from inorganic materials such as alumina, titanium dioxide, zirconia oxides, glassy materials, etc.
For gas separation applications, the membrane of choice is typically a polymeric membrane. One of the issues facing polymeric membranes, however, is their well-known trade-off between permeability and selectivity as illustrated by Robeson's upper bound curves (Robeson, J Membr. Sci. 1991, 62:165; Robeson, J Membr. Sci., 2008, 320:390-400). In particular, there is an upper bound for selectivity of, for example, one gas over another, such that the selectivity decreases with an increase in membrane permeability.
Metal-organic frameworks (MOFs) such as zeolitic imidazolate frameworks (ZIFs) have been previously incorporated into polymeric membranes to create mixed matrix membranes. The purpose of the use of MOFs was to increase the permeability of said membranes. These mixed matrix membranes were prepared by blending ZIFs with polymers, in which no chemical reaction between the ZIFs and the polymers occurred. This allowed for an increase in the permeability of the membranes, due to the poor interaction between the ZIFs and polymers at the polymer-zeolite interface. In particular, non-selective interfacial voids were introduced in the membranes such that the voids allowed for increased permeability but decreased selectivity of given materials. This has been referred to as a “sieve-in-a-cage” morphology (Hillock et al., Journal of Membrane Science. 2008, 314:193-199). FIGS. 1A-B illustrate prior art membranes exhibiting “sieve in a cage” morphology (Mahajan, et al., J Appl. Polym. Sci., 2002, 86:881).
Such “sieve-in-a-cage” morphology has resulted in mixed matrix membranes that fail to perform above a given Robeson upper bound trade-off curve. That is, a majority of such membranes fail to surpass the permeability-selectivity tradeoff limitations, thereby making them less efficient and more costly to use. As a result, additional processing steps may be required to obtain the level of gas separation or purity level desired for a given gas.